Evolution of Carbon Isotope Fractionation in Cyanobacteria

Abstract

The history of Earth’s carbon cycle reflects trends in atmospheric composition convolved with the evolution of photosynthesis. Fortunately, key parts of the carbon cycle have been recorded in the carbon isotope ratios of sedimentary rocks. The dominant model used to interpret this record as a proxy for ancient atmospheric CO₂ is based on carbon isotope fractionations of modern photoautotrophs, and longstanding questions remain about how their evolution might have impacted the record. We interrogated the intersection of environment and evolution by measuring both biomass (εp) and enzymatic (εRubisco) carbon isotope fractionations of a cyanobacterial strain (Synechococcus elongatus PCC 7942) solely expressing a putative ancestral Form 1B rubisco dating to >>1 Ga. This strain, nicknamed ANC, grows in ambient pCO₂ and displays larger εp values than WT, despite having a much smaller ε_(Rubisco) (17.23 ± 0.61‰ vs. 25.18 ± 0.31‰, respectively). Measuring both enzymatic and biomass fractionation revealed a surprising result—ANC εp exceeded ANC εRubisco in all conditions tested, violating prevailing models of cyanobacterial carbon isotope fractionation. However, these models were corrected by accounting for cyanobacterial physiology, notably the CO₂ concentrating mechanism (CCM). Our modified model indicated that powered inorganic carbon uptake systems contribute to ε_p, and this effect is exacerbated in ANC. These data suggested that understanding the evolution of both the CCM and rubisco is critical for interpreting the carbon isotope record, and that large fluctuations in the record may reflect the evolving efficiency of carbon fixing metabolisms as well as changes in atmospheric CO₂.